Ultra-selective uranium separation by in-situ formation of π-f conjugated 2D uranium-organic framework

With the rapid development of nuclear energy, problems with uranium supply chain and nuclear waste accumulation have motivated researchers to improve uranium separation methods. Here we show a paradigm for such goal based on the in-situ formation of π-f conjugated two-dimensional uranium-organic framework. After screening five π-conjugated organic ligands, we find that 1,3,5-triformylphloroglucinol would be the best one to construct uranium-organic framework, thus resulting in 100% uranium removal from both high and low concentration with the residual concentration far below the WHO drinking water standard (15 ppb), and 97% uranium capture from natural seawater (3.3 ppb) with a record uptake efficiency of 0.64 mg·g−1·d−1. We also find that 1,3,5-triformylphloroglucinol can overcome the ion-interference issue such as the presence of massive interference ions or a 21-ions mixed solution. Our finds confirm the superiority of our separation approach over established ones, and will provide a fundamental molecule design for separation upon metal-organic framework chemistry.

This work reported a unique route for uranium separation by means of a precipitation method.Interestingly, such precipitation method not only afforded ultrahigh uranium selectivity over other ions, but also high adsorption capacity, fast adsorption kinetics, high affinity, and good reusability.All these features confirmed its superiority over common adsorption method.Subsequently, upon such precipitation method, the author also observed outstanding performance in extraction of uranium from seawater.Based on these characterizations including in IR, XPS, PXRD, SEM-EDS, and TEM, the author proposed a mechanism of in-situ formation of π-f conjugated 2D uranium-organic framework (UOF).These results are exciting and preparation was carried out carefully.The manuscript is clearly written and well organized.Considering its novelty and the outstanding performance for uranium extraction, I consider this manuscript is appropriate for the Nature Communications after minor revision.For the content of the manuscript, I suggest that the author strengthen the exploration of the mechanism, e. g. ( 1) why is H3TFP ligand the best?what is relationship between uranium capture and organic ligand?who is the key factor?(2) why is the uranium selectivity better over other ions using H3TFP ligand?(3) Assuming such precipitation method with this mechanism of in-situ formation of π-f conjugated 2D UOF, however, we are not clear about the coordination number and bond length, which calls additional characterizations in this aspect.
For achieving uranium separation, different from the established methods relying on MOFs, COF, or POPs, the authors demonstrated in this work a facile approach of UOF-based approach using just an organic ligand.The results are very appealing, such as uranium theoretical uptake up to 1.6 g/g, high uranium capture capacity of 0.64 mg•g-1•d-1 from natural seawater, and high U selectivity over other metal ions.All these merits come from the unique design for such uranium separation by means of the in-situ formation of π-f conjugated 2D uranium-organic framework.This reviewer supports its publication in Nature Comm but revisions are needed to address the comments below.
1.The authors used five comparable ligands to fix uranium ions, and difference in the uranium uptake was obtained.However, no reason was given for such difference.It is suggested DFT calculations be performed to explain the difference, which could be instructive for new design.
2. Although the structure of UOF is clear and some strong evidences have been shown; however, additional evidences such as EXAFS are needed to further verify the structure of such UOF.
3. The author emphasized the advantage of such UOF method.How's the cost for uranium capture from seawater as compared with some established methods? 4. References are needed for some comparisons in the manuscript, e.g.page 9, line 213.
5. Units should be unified, for example in Fig. 3, ppm and mg/L are mixed.

Reviewer #2 (Remarks to the Author):
Uranium separation based on adsorption technology has always been a hot topic and challenge in current research.For example, there has developed several remarkable adsorbents, such as porous polymers (POPs), metal-organic frameworks (MOFs), and covalent organic frameworks (COFs).As stated by the author, these established adsorbents indeed need the first preparation of such materials, and next uranium separation can be executed.By contrast, the author in this work showed a distinct method with the just use of small functionalized organic molecule, 1,3,5-triformylphloroglucinol, but they can achieve a big outcome in uranium separation through an uncommon precipitation method.Whatever the adsorption capacity, adsorption rate, adsorption selectivity, as well as reuse of adsorbents and uranium extraction from seawater, all these indicate its advantage over current adsorbents.The mechanism is also clear and contributed by the in-situ formation of π-f conjugated 2D uranium-organic framework.Although I really appreciate this job, however, I think it still needs to be improved as follows.
There is no problem using 1,3,5-triformylphloroglucinol as an adsorbent.However, there lacks the data to support the solubility of 1,3,5-triformylphloroglucinol in the water.This is very important for such MOF-based route.
The U selectivity over Cu2+, Fe3+, Nd3+, Th4+, and VO3-ions are attractive.But there lack in-depth discussion in the reason.Especially for Cu2+ ions, it can also form d-f conjugated 2D Cu-organic framework.
Such precipitation method is interesting and effective.However, in light of the nature of 1,3,5-triformylphloroglucinol, can the author extend to liquid extraction by 1,3,5-triformylphloroglucinol?This will be also attractive for uranium separation, if the results are positive.
The current uranium extraction from seawater is exciting.However, I still concern about its adsorption capacity after one month, since the current adsorption capacity is till low, relative to other benchmark adsorbents.

Minor issues
The title is long.'Ultra-selective uranium separation by in-situ formation of π-f conjugated 2D uranium-organic framework' is more proper.
Page 1, line 9, 'has' should be 'have'.Please also check other grammar and spelling errors.
Page 6, there lacks a discussion to explain the pH effect.

Reviewer #3 (Remarks to the Author):
This work reported a unique route for uranium separation by means of a precipitation method.
Interestingly, such precipitation method not only afforded ultrahigh uranium selectivity over other ions, but also high adsorption capacity, fast adsorption kinetics, high affinity, and good reusability.
All these features confirmed its superiority over common adsorption method.Subsequently, upon such precipitation method, the author also observed outstanding performance in extraction of uranium from seawater.Based on these characterizations including in IR, XPS, PXRD, SEM-EDS, and TEM, the author proposed a mechanism of in-situ formation of π-f conjugated 2D uranium-organic framework (UOF).These results are exciting and preparation was carried out carefully.The manuscript is clearly written and well organized.Considering its novelty and the outstanding performance for uranium extraction, I consider this manuscript is appropriate for the Nature Communications after minor revision.
For the content of the manuscript, I suggest that the author strengthen the exploration of the mechanism, e. g. ( 1) why is H3TFP ligand the best?what is relationship between uranium capture and organic ligand?who is the key factor?(2) why is the uranium selectivity better over other ions using H3TFP ligand?(3) Assuming such precipitation method with this mechanism of in-situ formation of π-f conjugated 2D UOF, however, we are not clear about the coordination number and bond length, which calls additional characterizations in this aspect.

Response to reviewer #1:
Thank for your useful comments on improving the quality of our manuscript.According to your comments, we have carefully made a revised manuscript and supplementary, which are highlighted in red in the revision.
The point-to-point response is listed as follows.
Comment 1: The authors used five comparable ligands to fix uranium ions, and difference in the uranium uptake was obtained.However, no reason was given for such difference.It is suggested DFT calculations be performed to explain the difference, which could be instructive for new design.
Response: Thank you this comment.In the revision, DFT calculation was added, and the difference in uranium capture for these organic ligands was clearly explained by the binding energy, the coordination structure and coplanarity.And a discussion was added in the revision (see page 3-4, highlight in red, Supplementary Fig. 1).

Correction:
To clarify the difference in UO 2 2+ capture for these organic ligands, we then carried out the calculation on the binding energy (ΔG) of these organic ligands with UO 2 2+ ion by density functional theory (DFT) method.A planar six-coordination model of one UO 2 2+ ion coordinated by three these organic ligands was used to carried out DFT calculation.The optimized coordination structures of them were shown in Supplementary Fig. 1.The binding energy (ΔG) gives a hierarchy of H 4 THQ (-0.29 eV)>H 6 HTP (-0.69 eV)>H 4 EAA (-1.02 eV)>H 2 HPD (-2.99 eV)>H 3 TFP (-3.91 eV).Generally, negative binding energy (ΔG) suggests the reaction thermodynamically spontaneous, while this also obeys a rule, viz. the more negative, and the stronger the binding.Thus, the negative ΔG values mean that all these organic ligands can capture UO 2 2+ ion through coordination, which is consistent with the experimental results, while the smallest and biggest ΔG value in respectively H 3 TFP and H 4 THQ means the strongest and weakest binding and consequently the biggest and smallest UO 2 2+ uptake, which is also in good agreement with the experimental results.Moreover, the hierarchy in binding energy (ΔG) is also in accord with the hierarchy in the UO 2 2+ uptake, confirming the adsorption of UO 2 2+ ions by these organic ligand obeying the defined planar coordination principle.In addition, seen from the optimized coordination structures of them, it is found that the chelate coordination in the combination of one aldehyde oxygen and one hydroxyl oxygen is more beneficial for U-O coordination and planar coordination configuration such as H 4 TFP and H 2 HPD over the chelate coordination from two hydroxyl oxygens such as H 4 EAA, H 6 HTP, H 4 THQ, due to smaller steric hindrance in the combination of one aldehyde oxygen and one hydroxyl oxygen.And this could be the key to determine the UO 2 2+ uptake performance.Furthermore, big conjugated structure is also more beneficial for planar coordination configuration and consequently UO 2 2+ uptake, e.g.H 6 HTP and H 4 EAA vs. H 4 THQ.
Comment 2: Although the structure of UOF is clear and some strong evidences have been shown; however, additional evidences such as EXAFS are needed to further verify the structure of such UOF.
Response: Thank you this comment.In the revision, EXAFS was measured.The fitting results strongly suggests the planar six-coordination with two different types in the bond length of 2.42±0.02Å and 2.51±0.01Å (see Fig. 6 and Supplementary Figs. 13, 14, Table 4), which is in good agreement with the experimental results, thus, strongly supporting our previous results.

Correction:
In addition, to gain deep insight into the local coordination sphere of the uranium species, U K-edge X-ray absorption near-edge structure (XANES, Supplementary Fig. 13) and extended X-ray absorption fine structure (EXAFS, Supplementary Fig. 14) spectroscopy were performed.UO 2 (NO 3 ) 2 •6H 2 O was used as the reference standard.It was found that UOF-TFP rendered comparable XANES and EXAFS spectra with that observed in UO 2 (NO 3 ) 2 •6H 2 O, confirming their similarity in the valence state and coordination surrounding of uranium.As we know, uranium in UO 2 (NO 3 ) 2 •6H 2 O is hexavalent in the form of UO 2 2+ , and UO 2 2+ in UO 2 (NO 3 ) 2 •6H 2 O takes planar six coordination with two NO 3 -ions and two coordination water molecules, where each NO 3 -ion displays a chelate coordination mode with two NO 3 -oxygen atoms to fix one UO 2 2+ ion.
Accordingly, we can deduce also UO 2 2+ ions with also the planar six coordination in UOF-TFP (Supplementary  Comment 3: The author emphasized the advantage of such UOF method.How's the cost for uranium capture from seawater as compared with some established methods? Response: Thank you this comment.In the revision, a comparison of cost was comparied.The cost of preparing our adsorbent of H3TFP is as low as 1.3 $/g, which is far below the current benchmark adsorbents such as COF 4P (4.7 $/g, Angew.Chem.Int. Ed. 2023, 62, e202303129) and COF-4 (2.7 $/g, Nat.Common. 2023Common. , 14, 1106) ) Correction: Moreover, we should also consider the economy of uranium extraction from seawater.In generally, the manufacturing cost of adsorbents dominates the total cost for uranium extraction from seawater.For our case, the cost of preparing H 3 TFP adsorbent is as low as 1.3 $/g, confirming its economic feasibility.Such value is far below the current benchmark adsorbents such as COF 4P (4.7 $/g) 14 and COF-4 (2.7 $/g) 15 .
(Page 9) Comment 4: References are needed for some comparisons in the manuscript, e.g.page 9, line 213.
Response: Thank you this comment.In the revision, the references were added.

Correction:
The U-O bond length of 2.40-2.70Å is in the normal range. 33-34  (Page 11) Comment 5: Units should be unified, for example in Fig. 3, ppm and mg/L are mixed.
Response: Thank you this comment.In the revision, the units were unified in ppm unit (see the revised Fig. 3).

Response to reviewer #2:
Thank for your valuable comments.In light of these comments, we have carefully made a revised manuscript and supplementary, which are highlighted in red in the revision.
The point-to-point response is listed as follows.
Comment 1: There is no problem using 1,3,5-triformylphloroglucinol as an adsorbent.However, there lacks the data to support the solubility of 1,3,5-triformylphloroglucinol in the water.This is very important for such MOF-based route.
Response: Thank you this comment.In the revision, the solubility of 1,3,5-triformylphloroglucinol in the water was tested.Our results strongly suggest the insolubility of 1,3,5-triformylphloroglucinol ligand in water (see Supplementary Figs. 6 and 7), thus fully meeting our demand as solid adsorbent.

Correction:
In both solid-and liquid-extraction routes, a key step in the desorption process is the solid-liquid separation between UO 2 (NO 3 ) 2 solution and H 3 TFP precipitation regeneration from desorption.Thus, it is important to reveal the solubility of H 3 TFP in water.As shown in Supplementary Fig. 6, it was clear that H 3 TFP is completely soluble in ODCM, but insoluble in water.This can be further attested by UV-visible spectral test (Supplementary Fig. 7), where H 3 TFP in ODCM gave a strong adsorption peak at 320 nm, while no obvious adsorption was observed for H 3 TFP in water.Comment 2: The U selectivity over Cu 2+ , Fe 3+ , Nd 3+ , Th 4+ , and VO3 -ions are attractive.But there lack in-depth discussion in the reason.Especially for Cu 2+ ions, it can also form d-f conjugated 2D Cu-organic framework.
Response: Thank you this comment.In the revision, we make a clear explain on the selectivity of U ion over other ions.First, the selectivity of UO2 2+ ion over Fe 3+ , Nd 3+ , Th 4+ , and VO3 -ions is determined by their coordination feature, viz.planar coordination of UO2 2+ ion vs. spherical coordination of Fe 3+ , Nd 3+ , Th 4+ , and VO3 -ions, since only planar coordination can meet the demand of formation of π-d/f conjugated 2D metal-organic framework (the uranium adsorption mechanism).Second, to disclose the selectivity of UO2 2+ ion over planar-coordinated Cu 2+ ions, we further carried out DFT calculation (Supplementary Fig. 3).And we found that the binding energy between H3TFP and UO2 2+ ions is far bigger than that between H3TFP and Cu 2+ ion, strongly suggesting higher affinity of H3TFP towards UO2 2+ over Cu 2+ ion and consequently UO2 2+ selectivity over Cu 2+ ion.

Correction:
The selectivity of UO 2 2+ over Fe 3+ , Nd 3+ , Th 4+ , and VO 3 -can be easy to understand, since UO 2 2+ is planar coordinated and can effectively construct the π-f conjugated 2D uranium-organic framework, whereas other ions such as Fe 3+ , Nd 3+ , Th 4+ , and VO 3 -are spherical coordinated and can not construct corresponding π-d/f conjugated 2D metal-organic framework.However, Cu 2+ ion also own the planar coordination feature, and can also form the π-d conjugated 2D metal-organic framework, which will theoretically result in considerably competitive adsorption with UO 2 2+ ion.Thereby, to understand the UO 2 2+ /Cu 2+ selectivity in this work, DFT calculation on binding energy was carried out (Supplementary Fig. 3).The negative binding energy (ΔG) of -1.16 eV implies the adsorption potential of H 3 TFP ligand towards Cu 2+ ions.But the binding towards Cu 2+ ion is far weaker than that towards UO 2 2+ ion, as evidenced by the ΔG value (-1.16 eV) in Cu 2+ ion that is far bigger than that (-3.91 eV) in UO 2 2+ ion; thus H 3 TFP ligand can enable selective UO 2 2+ capture over Cu 2+ ion.
(Page 5-6) Supplementary Fig. 3| View of the optimal structure from DFT calculation for the coordination structure of Cu(II) ion with H 3 TFP ligands.
Comment 3: Such precipitation method is interesting and effective.However, in light of the nature of 1,3,5-triformylphloroglucinol, can the author extend to liquid extraction by 1,3,5-triformylphloroglucinol?This will be also attractive for uranium separation, if the results are positive.
Response: Thank you this comment.In the revision, we further explored the liquid extraction by using 1,3,5-triformylphloroglucinol as liquid phase extractant.Interestingly, using 1,3,5-triformylphloroglucinol dissolved in various organic solvents as liquid phase extractant, we found that such liquid extraction from a 50 ppm U solution can be also effective (Supplementary Fig. 5 and revised Fig. 4).Especially, organic solvents of dichloromethane (DCM) and ortho-dichlorobenzene (ODCB) can lead to 100% removal within 15 min, which is equal to that observed in the solid-extraction route.Extending the contacting time up to 48 h, the uranium concentration was reduced as low as 1.24 ppb, far below the WHO standard of drinking water.
These results strongly support its promising potential in liquid extraction.
Correction: Interestingly, although H 3 TFP is insoluble in water, however it gave good solubility in many organic solvents such as dichloromethane (DCM), toluene (PhMe), ortho-dichlorobenzene (ODCB), p-xylene (PX), m-xylene (MX), ortho-xylene (OX), and trimethylbenzene (TB).Thus, we further developed the liquid-extraction route (Fig. 4) and the results were shown in Supplementary Fig. 5. Notably, such liquid-extraction route was found to be also effective for uranium capture.Especially, H 3 TFP in DCM and ODCB was found to gave 100% removal for a 50 ppm uranium solution within 15 min.This result is comparable with that observed in the solid-extraction route.We also extended the contacting time up to 48 h for H 3 TFP in DCM, and found extremely low residual concentration of uranium (1.24 ppb), which is also far below the WHO standard of drinking water.
(Page 8) Comment 4: The current uranium extraction from seawater is exciting.However, I still concern about its adsorption capacity after one month, since the current adsorption capacity is till low, relative to other benchmark adsorbents.
Response: Thank you this comment.In the revision, we further explored the uranium extraction from seawater under long contacting time.Impressively, we found that extending contacting time to extending contacting time to 14 days did not increase uptake capacity.Seen from Supplementary Table 3, the new results of 6.4 mg/g uptake for our case is clearly located on the top level in all established materials for such use.

Correction:
In addition, we also conducted the uranium extraction under a longer contacting time from a 20 L natural seawater (3.3 ppb U) using 10 mg H 3 TFP adsorbent.Impressively, almost 97% uranium can be captured after 10 days, giving uranium uptake as high as 6.4 mg/g, while extending contacting time up to 14 days did not increase uranium uptake.A comparison in uranium uptake between established materials and our case is shown in Supplementary Table 3, which clearly suggests our case with the location of top level in the field of uranium extraction from seawater by adsorbents.
(Page 9) Comment 5: The title is long.'Ultra-selective uranium separation by in-situ formation of π-f conjugated 2D uranium-organic framework' is more proper.
Response: In the revision, the title was corrected as 'Ultra-selective uranium separation by in-situ formation of π-f conjugated 2D uranium-organic framework'.
Comment 6: Page 1, line 9, 'has' should be 'have'.Please also check other grammar and spelling errors.
Response: In the revision, we carefully checked the manuscript, and all these grammar and spelling errors were completely corrected.
Comment 7: Page 6, there lacks a discussion to explain the pH effect.
Response: Thanks for this comment.In the revision, a discussion of pH effect was added.
Correction: High acidity leading to a sharp decrease in uranium uptake is mainly due to the protonation that will significantly affect the coordination of H 3 TFP hydroxyl groups, and high alkalinity resulting in a sharp decrease in uranium uptake is mainly due to the solution of H 3 TFP under such condition that will significantly affect the UOF formation surrounding.

Response to reviewer #3:
Thank for your helpful comments.In light of these comments, we have carefully made a revised manuscript and supplementary, which are highlighted in red in the revision.
The point-to-point response is listed as follows.
Comment 1: why is H3TFP ligand the best?what is relationship between uranium capture and organic ligand?who is the key factor?
Response: Thank you this comment.To make clear the above questions, in the revision we further made DFT calculation on these ligands.The biggest binding energy between UO2 2+ and H3TFP explained the best uranium capture performance in H3TFP.The negative binding energy for all these ligands strongly suggests the adsorption thermodynamically spontaneous in a mode of formation of π-f conjugated 2D uranium-organic framework.A comparison in binding energy, the coordination structure and coplanarity, it was found that the chelate coordination in the combination of one aldehyde oxygen and one hydroxyl oxygen is more beneficial for U-O coordination and planar coordination configuration, which could be the key to determine the UO2 2+ uptake performance.In addition, big conjugated structure is also more beneficial for planar coordination configuration and consequently UO2 2+ uptake.And a discussion was added in the revision (see page 3-4, highlight in red, Supplementary Fig. 1).shift disappear Fig. 6| EXZAF of UOF-TFP.a Experimental EXZAF data (red) and fitting results (black).b View of coordination surrounding of UOF-TFP and UO 2 (NO 3 ) 2 •6H 2 O, where U-O ax represents the U=O coordination bonds, U-O eq represents the U-O nitrate plus U-O water in UO 2 (NO 3 ) 2 •6H 2 O and U-O hydroxyl plus U-O aldehyde in UOF-TFP.

Fig. 3|
Fig. 3| Uranium adsorption upon H 3 TFP.a Adsorption kinetics from 50 ppm U solution upon H 3 TFP adsorbent.b Adsorption kinetics from 1 ppm U solution upon H 3 TFP adsorbent.c Adsorption capacity of H 3 TFP adsorbent.

(Page 9 )
Supplementary Fig.6|A comparison of the solution of H 3 TFP in water and ODCM.Clearly, H 3 TFP is insoluble in water, but completely soluble in ODCM. in ODCM Supplementary Fig.7|A comparison of UV-visible spectral for H 3 TFP in water and ODCM.This also clearly suggests that H 3 TFP is insoluble in water, but soluble in ODCM.

Fig.
Fig. 4| Our MOF routes.View of the adsorption-desorption cycle by means of our new separation approach in both solid-and liquid-extraction routes.

Table 4 )
. High-quality fits of the EXAFS data for UOF-TFP (Figs.6a and 6b) strongly suggest an eight coordination of uranium, where two U-O coordinations with bond length of 1.85±0.01Å are assigned to U=O bond of UO 2 2+ ion, other six U-O coordinations with longer bond length are assigned to the planar U-O coordination from TFP 3-ligand.This also reveals two types of U-O bond with equal numbers in the bond length of 2.42±0.02Å and 2.51±0.01Å, respectively.We then comparied the results from EXAFS data with the results from structural simulation, and found that the U=O bond of UO 2 2+ ion form EXAFS data is well consistent with the results from structural simulation (1.84 Å), while the bond length of 2.42±0.02Å and 2.51±0.01Å from